The invention relates to an optical identification element, especially for counterfeit protection and copy protection, comprising at least one optically anisotropic layer, which has at least two regions having different preferred directions, and a layer that is at least partially reflective.
The demands made of the protection of items such as banknotes credit cards, securities, identity cards and the like against counterfeiting are constantly increasing as a result of the availability of high quality copying techniques. In addition, pirated copies of branded products, such as compact discs. computer software, electronic chips and so on, are increasingly being produced and distributed worldwide. On account of the growing number of instances of counterfeiting there is accordingly a great need for new, counterfeit-protected elements that can be identified both visually and by machine.
In recent years, new types of counterfeit-protected identification elements that utilise liquid crystals and associated optical effects have become known. Examples thereof are described in EP-A-0 689 065 and WO-A-98/52077, the disclosures of which are hereby incorporated by reference.
Such identification elements are based on a hybrid layer structure which comprises a layer having molecules that are fixed (crosslinked) in the liquid-crystalline state, which layer is optically anisotropic and usually acts as an optical retarder. In the production of that layer the orientation of the liquid crystal molecules can be preset to be different in different areas, preferably by means of a correspondingly structured orientation layer, such as can be produced, for example, from a linearly photopolymerised polymer network. The result is an optically structured, fixed liquid crystal layer having an orientation pattern that has been determined in advance. When viewed without an additional aid, the orientation pattern, i.e. the information thus recorded in the liquid crystal layer, is initially invisible. The layers appear in this case to be transparent. The orientation pattern becomes visible only when the identification element is placed between two polarisers.
In especially suitable arrangements of such counterfeit-protected identification elements, the optically structured liquid crystal layer is located over a reflective background. This makes it possible for the orientation pattern, i.e. the corresponding information, to be rendered visible using only one polariser which is held above the element.
As the reflective background (referred to hereinafter as the reflector) there have been used hitherto materials such as diffuse or mirroring metal surfaces, reflective lacquers, reflective paper or the like. In that case the mode of operation of the identification elements usually requires the incident light to retain its polarisation state, at least to a certain extent, on reflection at the reflector.
A disadvantage of the reflectors known hither to for the said identification elements is that brightness and contrast are lost to a comparatively great extent on reflection.
According to the invention it is therefore proposed to use a holographic reflector. This type of reflector, known per se, reflect (and focuses) the ambient light into a predetermined cone of diffuse light and thus produces a substantial increase in brightness in the region of the cone. At the same time, dazzling reflections (glare) are largely avoided and an excellent brightness distribution is obtained. In addition, the polarisation is almost completely retained. These properties bring about a crucial improvement in the visual appearance of the identification elements, which is important both for the actual identification and its ability to be discerned and for market acceptance. Thus, according to the invention, there is provided optical identification element, especially for counterfeit protection and copy protection, comprising at least one optically anisotropic layer , which has at least two regions having different preferred directions, and a layer that is at least partially reflective, characterised in that the at least partially reflective layer is a holographic reflector.
Surprisingly, tests have shown that holographic reflectors, despite their complex structure, do not have an adverse effect on the mode of action of the identification element, which will usually likewise be of comparatively complex structure.
The reflector is preferably based on an achromatic, or white light, hologram, that is to say the predetermined cone of diffuse light produced by the holographic reflector is substantially white.
For the sake of simplicity the opening explanation, of the structure and principle of operation of counterfeit-protected identification elements having an optically structured, fixed liquid crystal layer was limited to basic principles. However, a large number of arrangements that are more complex in terms of structure and optical mode of action exist, some of which are described in the publications mentioned above, and it is possible to imagine other arrangements operating in accordance with the same basic principles. For example, it is possible additionally to integrate one or more polariser layers, for example in the form of a liquid crystal layer containing dichroitic dyes or in the form of a conventional polarisation film, it also being possible for a polariser to contribute to the information recorded in the element when the polarisation properties are different in different areas. Also possible are combinations of two or more information-carrying liquid crystal layers and optionally associated polariser layers, or alternatively configurations having cholesteric filters, especially for producing different colours. It will be clear to the person skilled in the art that all those arrangements and similar arrangements of counterfeit-protected identification elements, where they are used in reflection, will benefit from the above-mentioned advantages and are covered by the present invention.
According to a special embodiment of the invention, the holographic reflector is transflective or partially transparent. This enables, for example, an identification element according to the invention to be mounted over a visible feature, which nevertheless remains visible as a result of the partial transparency of the reflector.
Preferably, the optically anisotropic layer is formed with liquid crystal molecules that are substantially fixed in their orientation. The layer may be formed with a layer of crosslinked liquid-crystalline monomers, oligomers or polymers. An orientation layer may be provided for this liquid crystal molecules, preferably formed from a linearly photopolymerised polymer network.
The invention extends to an item such as a banknote or identity card with protection against forgery and/or copying, characterised by an optical identification element as set forth above.
A method of checking such an element for authenticity may comprise passing light through a polariser onto the optical component and viewing the reflected light from this layer through the polariser.